1. Field of the Invention
The present invention relates to a non-aqueous electrolyte battery system and more particularly to a non-aqueous electrolyte battery system capable of detecting that an electrolyte is frozen.
2. Description of Related Art
A non-aqueous electrolyte secondary battery, typified by a lithium ion secondary battery, is configured such that a positive electrode sheet, a negative electrode sheet, and a non-aqueous electrolyte are contained in a battery case. During charge/discharge of the lithium ion secondary battery, electric charges are transferred between the positive electrode sheet and the negative electrode sheet through the non-aqueous electrolyte. Herein, when the lithium ion secondary battery is placed at an environmental temperature below a freezing point of the non-aqueous electrolyte, the non-aqueous electrolyte may be frozen in some cases.
While the non-aqueous electrolyte freezes, it is preferable to restrict the use of the lithium ion secondary battery. The reason is because, for instance, when charging of the lithium ion secondary battery is carried out while the non-aqueous electrolyte is frozen, lithium may be deposited on the negative electrode sheet, leading to deterioration of battery performance of the battery. Accordingly, a system for controlling the use of the lithium ion secondary battery is preferably arranged to appropriately detect that the non-aqueous electrolyte is frozen.
For instance, Japanese patent publication application No. 2006-155916 discloses a technique for detecting that the electrolyte is in a frozen state when an open circuit voltage of the battery rises while the battery temperature decreases.
Meanwhile, the foregoing conventional technique is a technique for detecting that the electrolyte is in the frozen state at the time when freezing of the electrolyte occurs. Specifically, the aforementioned conventional technique is not directed to detecting whether or not the state of the electrolyte at any given point in time is a frozen state. However, freezing of the non-aqueous electrolyte is likely to occur during a non-use period in which charging and discharging of a lithium ion secondary battery are not performed. It is therefore preferable to appropriately detecting whether or not the non-aqueous electrolyte is in a frozen state at the time when the use of the lithium ion secondary battery is enabled to start after the non-use period.
It is therefore conceivable for example to detect the frozen state of the non-aqueous electrolyte when becomes frozen and further estimate whether or not the frozen state has been eliminated based on for example a temperature transition and an elapsed time from when the electrolyte freezes. However, for the non-aqueous electrolyte, the time required up to freezing and the time required from freezing to melting may be different. Further, depending on a residual charging capacity and others, the temperature and others at which freezing and melting are caused may be different. Therefore, for the method for detecting the time when the non-aqueous electrolyte becomes frozen, it is unlikely to accurately detect whether or not the state of the non-aqueous electrolyte at any given point in time corresponds to the frozen state.
The present invention has been made to solve the foregoing problems of the conventional techniques and has a purpose to provide a non-aqueous electrolyte battery system capable of accurately detecting that a non-aqueous electrolyte of a non-aqueous electrolyte secondary battery is in a frozen state.